Directional control has become increasingly important in the drilling of subterranean oil and gas wells, for example, to more fully exploit hydrocarbon reservoirs. Downhole steering tools, such as two-dimensional and three-dimensional rotary steerable tools, are commonly used in many drilling applications to control the direction of drilling. Such steering tools commonly include a plurality of force application members (also referred to herein as blades) that may be independently extended out from and retracted into a housing. The blades are disposed to extend outward from the housing into contact with the borehole wall. The direction of drilling may be controlled by controlling the magnitude and direction of the force or the magnitude and direction of the displacement applied to the borehole wall. In rotary steerable tools, the housing is typically deployed about a shaft, which is coupled to the drill string and disposed to transfer weight and torque from the surface (or from a mud motor) through the steering tool to the drill bit assembly.
In general, the prior art discloses two types of directional control mechanisms employed with rotary steerable tool deployments. U.S. Pat. Nos. 5,168,941 and 6,609,579 to Krueger et al disclose examples of rotary steerable tool deployments employing the first type of directional control mechanism. The direction of drilling is controlled by controlling the magnitude and direction of a side (lateral) force applied to the drill bit. This side force is created by extending one or more of a plurality of ribs (referred to herein as blades) into contact with the borehole wall and is controlled by controlling the pressure in each of the blades. The amount of force on each blade is controlled by controlling the hydraulic pressure at the blade, which is in turn controlled by proportional hydraulics or by switching to the maximum pressure with a controlled duty cycle. Krueger et al further disclose a hydraulic actuation mechanism in which each steering blade is independently controlled by a separate piston pump. A control valve is positioned between each piston pump and its corresponding blade to control the flow of hydraulic fluid from the pump to the blade. During drilling each of the piston pumps is operated continuously via rotation of a drive shaft.
U.S. Pat. No. 5,603,386 to Webster discloses an example of a rotary steerable tool employing the second type of directional control mechanism. Webster discloses a mechanism in which the steering tool is moved away from the center of the borehole via extension (and/or retraction) of the blades. The direction of drilling may be controlled by controlling the magnitude and direction of the offset between the tool axis and the borehole axis. The magnitude and direction of the offset are controlled by controlling the position of the blades. In general, increasing the offset (i.e., increasing the distance between the tool axis and the borehole axis) tends to increase the curvature (dogleg severity) of the borehole upon subsequent drilling. Webster also discloses a hydraulic mechanism in which all three blades are controlled via a single pump and pressure reservoir and a plurality of valves. In particular, each blade is controlled by three check valves. The nine check valves are in turn controlled by eight solenoid controlled pilot valves. Commonly assigned U.S. Pat. No. 7,204,339 employs hydraulic actuation to extend the blades and a spring biased mechanism to retract the blades. Spring biased retraction of the blades advantageously reduces the number of valves required to control the blades. The '339 application is similar to the Webster patent in that only a single pump and/or pressure reservoir is required to actuate the blades.
The above described steering tool deployments are known to be commercially serviceable. Notwithstanding, there is room for improvement of such tool deployments, especially for smaller diameter steering tool embodiments. For example, there is a need for improved caliper measurements for accurately determining the position of the center of the borehole and the diameter of the borehole in steering tools employing the above-described second type of directional control mechanism.